Michael Cussen asked:

> I have a six layer board (FR4) that has been setting on the shelf for three
> years, and not stored in  plastic bags.
> What would be your recommendation to assure that these boards would solder
> properly, when processed?
>
>
> Thanks In Advance
> Michael Cussen
> Medar Inc.
> mcussen@medar

1) Scrap two units,
1a) [assumes an SMT process is expected, disregard if not] print paste and
reflow without parts

1b) wave solder unit it as-is. (IMO baking is not going to help anyway, and
you want to see the worst case)

2) examine test units carefully, see that the wetting of the metal in the
wave is OK, and look for excessive "pooling" of the solder. The solder from
HASL or plating is almost bound to melt in a wave solder bath. If there is
a very thick layer of intermetallic Cu3Sn/Cu6Sn5 the effect is that both
the solder added by the bath and the metal on the board will not smoothly
and uniformly wet at the intermetallic. A similar but much more pronounced
effect (which I think is unlikely) would be evident if the copper below the
tin-lead were oxidized. Another bad indication is that you'd see blowholes,
etc. from having a lot of moisture in the laminate.

Based on what you see, you have several courses of action:

a) go ahead and process as usual. Bake only if you have blowholes, as this
would have a negative effect on solder wetting.

a1) If the wetting seems to be moderately affected on only the part printed
with paste and reflowed; try this if you have a wave solder process with OA
flux: turn your solder pump off, and slow conveyor speed somewhat to
achieve 100 C and longer duration. Put the boards through fluxing with OA
and "preheating" only. Wash/dry thoroughly and see if tin oxides are not
reduced upon subsequent SM processing.

b) send the PWB's out for a repeat HASL treatment, you might see your
legend ink darkened. This course of action would be primarily if you intend
to reflow SM parts with paste. Otherwise, your wave will have much the same
effect.

c) scrap 'em  and order again.

d) go see your inventory/production control people, give them your best
impression of a raving lunatic metallurgist. Threaten to report them to
APICS. Wrap the manager in a large 3 mil poly bag. This may inhibit
recurrences.

I've seen quite a few 3 year old boards soldered w/o much problem, but
usually stored carefully in poly bags. If you had them in cardboard boxes,
I would expect to see very poor wetting in a reflow operation. A wave
solder machine is going to dissolve and dilute all the existing tin-lead in
the machine's pot, a reflow operation will dilute it only to the extent of
the added paste. Oxides and sulfides will not dissolve, but they may be
physically removed during a wave operation.


regards,

Jerry Cupples
Interphase Corporation
Dallas, TX USA
http://www.iphase.com/